Destructable terrain in as2?

How can I make destructible terrain for a platform game in AS2? I have my own forum of it, where the mouse pointer is the gun and whenever it hits the ground it deletes the movieclip within the ground. The problem with this though is that the movieclips must only be a few pixels in size for it too look good, and it would take millions of them, making the game lag like h*ll. Actually all I need to know is how to erase part of the movieclip without a mask.